<style>
#scroller {
    overflow: scroll;
    width: 200px;
    height: 200px;
    box-sizing: border-box;
}

#scroller-outline {
    will-change: transform;
    outline: 5px solid rgba(0, 255, 0, 0.6);
    outline-offset: -4px;
    position: absolute;
    width: 200px;
    height: 200px;
}

.spacer {
    width: 300px;
    height: 300px;
}
</style>
<!-- #scroller outline should not be covered up by scrolling content background.
    It is achieved by painting the outline into the decoration layer which draws
    on top of all other layers. -->
<div id="scroller-outline"></div>
<div id="scroller">
    <div class="spacer"></div>
</div>

